1. Field of the Invention
Embodiments of this invention relate generally to actuator arms and head suspension assemblies (HSAs) for suspending and positioning read/write heads over magnetic media of the type generally used for storing digital data, and in particular embodiments to apparatus for preventing slippage of multiple stacked actuator arms and HSAs with respect to each other when mechanical shock is encountered, and systems incorporating the same.
2. Description of Related Art
Modem computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. To access memory locations on a hard disk, a read/write head is positioned slightly above the surface of the hard disk while the hard disk rotates beneath the read/write head at an essentially constant velocity. By moving the read/write head radially over the rotating hard disk, all memory locations on the hard disk can be accessed. The read/write head is typically referred to as "flying" head because it is coupled to a slider aerodynamically configured to hover above the surface on an air bearing located between the hard disk and the slider that forms as the hard disk rotates at high speeds.
In conventional disk drives, multiple hard disks are coupled to and rotate about a spindle, each hard disk presenting two substantially flat surfaces for reading and recording. Typically, these rotating hard disks are stacked in a parallel relationship within minimal spacing between them. Accordingly, the read/write heads must be designed to move within the narrow space between adjacent hard disks and fly close to the hard disk surfaces. To achieve this positional capability, the read/write heads in typical disk drives are coupled to the distal end of thin, arm-like structures called head suspension assemblies (HSAs), which are inserted within the narrow space between adjacent hard disks. These HSAs are made of materials and thicknesses as to be somewhat flexible and allow a measure of vertical positioning as the read/write head hovers over the surface of the rotating hard disk.
Each HSA is coupled at its proximal end to a rigid actuator arm that horizontally positions the HSA and read/write head over the hard disk surface. In conventional disk drives, all actuator arms are machined from a single piece of material, forming an multi-arm actuator assembly which moves as a unit under the influence of a voice coil motor to simultaneously position all HSAs and corresponding read/write heads over the hard disk surfaces.
As disk drives have become physically smaller in size with increased data storage capacity, hard disk data recording densities have increased dramatically and data tracks have become smaller and have been positioned increasingly closer together. Read/write heads and sliders have also seen a corresponding decrease in size. This decrease in size has made disk drive assemblies more sensitive and susceptible to manufacturing tolerances and assembly variations. However, manufacturers have found that these manufacturing tolerances and assembly variations can be minimized or mitigated by assembling and testing at sub-assembly levels rather than assembling and testing a disk drive in its entirety. In addition, because the smaller geometries of today's disk drives require costly, precision-made parts manufactured to exacting standards, manufacturers have found that the ability to couple and decouple sub-assemblies can substantially reduce costs should a part need rework or become irreparably damaged and require replacement.
Thus, it is presently desirable to manufacture individual actuator arms rather than a plurality of actuator arms machined from a single piece of material, and assemble and test a sub-assembly comprised of an actuator arm, HSA, and read/write head. Once testing of the sub-assembly is complete, the actuator arms can then be coupled together as a completed actuator assembly.
However, sub-assemblies can also increase the possibility of positional errors by introducing additional interfaces requiring accurate alignment. For example, if actuator arms are separately formed, they must eventually be coupled together by a bolt or bearing cartridge to form an actuator assembly. If the actuator assembly should encounter high physical shocks arising from non-operational conditions such as the dropping or bumping of the disk drive, the coupled actuator arms may slip with respect to each other. In addition, operational conditions such as crash stops may induce similar slippage of the coupled actuator arms. A crash stop occurs when the disk drive loses servo and the read/write heads are abruptly moved to the landing zone to avoid having the heads touch down onto data areas of the hard disk. As the read/write heads are "parked," the actuator arms encounter a crash stop pin designed to prevent the HSA from contacting the spindle. The abrupt stoppage of the actuator assembly against the crash stop pin may cause slippage of the coupled actuator arms. Such slippage is likely to cause data errors in present-day small geometry and positional error-intolerant disk drives.
The interface between the actuator arms and the HSAs is another area of possible slippage caused by the mechanical shocks discussed above. Such slippage is equally likely to cause data errors in present-day disk drives. One proposal for fastening actuator arms to HSAs is disclosed in U.S. Pat. No. 4,912,583 to Hinlein, incorporated herein by reference. The patent discloses a threaded clamp having a thin nut plate formed with a threaded boss and a screw for threadable engagement with the boss. The boss is formed to telescopically engage respective openings formed in the actuator arm and HSA, with the screw confining the components therebetween. The actuator arm and HSA are then clamped in vertical relationship such that the HSA and actuator arm act as a single unit. Because of the multiple plate and screw arrangement, the overall vertical profile is relatively high. Furthermore, because of the compressive force needed to keep the HSA and actuator arm in proper alignment, the geometry of the threaded clamp cannot be made arbitrarily small. The high vertical profile of the threaded clamp solution may therefore have trouble fitting into the low profile disk drives of today.
An alternative to the solution proposed above is disclosed in U.S. Pat. No. 4,829,395 to Coon, and allegedly improved upon in U.S. Pat. No. 5,172,286 to Jurgenson, both incorporated herein by reference. These patents teach a fastener for connecting the actuator arm to the HSA through use of a swaging process. The fastener, or swage mount, comprises a thin base plate formed on one side with an opening and formed on the other side with an outwardly projecting cylindrical boss of a predetermined height and radius corresponding to the opening formed in the HSA. The boss includes an inner engagement surface axially aligned with the base plate opening for receiving staking during the swaging process. Fastening is accomplished by first welding the swage mount base plate to the actuator arm, then positioning the HSA opening telescopically over the swage mount boss. An oversized swaging element is then staked through the openings to radially expand the swage mount boss to connect the components through an interference fit. The primary advantage of the swage mount fastener is the omission of the screw, which undesirably increases the vertical profile of the actuator arm to load beam joint. Disconnection of the components is easily accomplished by simply breaking the bond through application of a force exceeding the torque retention of the swage mount.
The asserted improvement in Jurgenson further expands on the principles of the swage mount disclosed by Coon by constructing two low profile swage mounts having complementarily formed hubs that, when swaged together, are said to provide sufficient torque retention to fasten the HSA to the actuator arm.
While the conventional swage mount configurations disclosed in Coon and Jurgenson allow a substantial reduction in overall vertical profile for the actuator arm/HSA connection, such swage mounts are subject to minimum profile limits in order to maintain sufficient torque retention and avoid slippage, which is primarily dependent upon hub height and hub radial thickness. Again, the high vertical profile of these solutions may be problematic in the low profile disk drives of today.